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ESA 2 > MSK 5a: dermatomes, myotomes and segmental innervation of limbs > Flashcards

MSK 5a: dermatomes, myotomes and segmental innervation of limbs Flashcards

1
Q

Describe the segmentation of the CNS

A

Segmented throughout

Each neural segment corresponds to a neural level

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2
Q

What does the embryological neural tube contain?

A

Precursor cells of the nervous system:

  • paired somites (W4: 44 pairs form from paraxial mesoderm, diminishes to about 35 pairs)
  • scelrotome (ventral): vertebrae and ribs
  • dermatomyotome (dorsal): dermis and muscle
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3
Q

How does the dermatomyotome develop?

A

Develops in association with a specific neural level of spinal cord (e.g. C6)
Takes the nerve supply with them from the neural tube as a spinal (segmental) nerve, e.g. C6
Skin and muscle derived from a single DMT have a common spinal nerve supply

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4
Q

What is the purpose of spinal nerve roots?

A

Connect each spinal nerve to a “segment” of cord (the cord is continuous)

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5
Q

What is the difference between dorsal (posterior) and ventral (anterior) spinal nerve roots?

A

Dorsal: contain only afferent (sensory) nerve fibres. Dorsal root ganglion is the cell bodies of the sensory neurones from the periphery
Ventral: contain only efferent (motor) and autonomic nerve fibres

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6
Q

General structure of spinal nerves?

A

Parallel bundles of axons encased in connective tissue
Mix of afferent and efferent: ventral and dorsal roots unite in the interventricular foramen
Interventricular foramen marks the division between the CNS and the PNS

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7
Q

General structure of nerves from inner to outer?

A

Endoneurium
Perineurium
Epineurium

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8
Q

How many pairs of spinal nerves are there and how are they numbered?

A

31

Numbered according to the level of vertebral column from which they emerge

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9
Q

Vertebral segments

A

1 vertebra=1 vertebral segment
Differentiate from sclerotomes
Spinal cord runs through vertebral foramina:
-multiple vertebral foramina=spinal canal
-spinal (segmental) nerves leave spinal cord via the intervertebral foramina to become peripheral nerves

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10
Q

Why are nerves referred to as either motor or sensory?

A

Relate to the majority of fibre types conveyed by that nerve: often have a small number of the other type too, e.g. nerves supplying muscles are mainly motor but have some sensory fibres to convey pain and proprioception

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11
Q

Describe the structure of the spinal cord

A

Starts at the inferior margin of the MEDULLA OBLONGATA
Terminates as the CONUS MEDULLARIS at L2

Much shorter than vertebral column. Most spinal cord segments are NOT vertically-aligned with the corresponding vertebrae

Long roots from inferior segments (lumbar/sacral/coccygeal) descend in CAUDA EQUINA to exit at their respective foramina

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12
Q

Where do the spinal nerves emerge?

A
  • 1st cervical pair emerge between the occipital bone and atlas (C1)
  • C1-C7 exit above the corresponding vertebrae
  • T1-L5 exit below the corresponding vertebrae
  • S1-S4 exit via 4 pairs of sacral foramina
  • S5 and Co1 exit via sacral hiatus (posterior)
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13
Q

What are rami?

A

Divisions of mixed spinal nerves

Rami communicantes-component of the ANS

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14
Q

What does the posterior/dorsal ramus supply?

A

Deep muscles and skin of the dorsal trunk (small)

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15
Q

What does the anterior/ventral ramus supply?

A

Muscles and skin of the upper and lower limbs and the lateral and ventral trunk (large)

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16
Q

Meningeal branch?

A

Spinal nerves also give off a meningeal branch
This re-enters the spinal cord through the intervertebral foramen
Supplies vertebrae, ligaments, blood vessels and meninges

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17
Q

Structure of dorsal/posterior rami

A

Divide again into medial and lateral branches
Supply the skin on the back in a “tidy” segmental manner
Narrow strip of muscle/skin in line with intervertebral foramen: supply synovial joints of vertebral column and deep muscles of back

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18
Q

Structure of ventral/anterior rami

A

Complicated
Enter the lumbar and sacral plexuses
Gap in between C4 and T2

19
Q

Where is the lumbar plexus located and what are the nerve roots?

A

Forms behind psoas muscles and emerges medial and lateral to psoas major

20
Q

Look at video about how to draw lumbosacral plexus to understand properly

A

Do it :)

21
Q

Major branches that run medial to the psoas muscles and their roots?

A 
Obturator (L2-4)
Lumbosacral trunk (L4,5)
22
Q

Major branches that run lateral to the psoas muscles and their roots?

A

Femoral (L2-4)
Iliohypogastric (L1, contribution of T12)
Ilioinguinal (L1)
Lateral femoral cutaneous (L2,3)

23
Q

Where does the sacral plexus form and out of what?

A

Within the pelvic cavity on the posterior pelvic wall, but interior to piriformis. Nerves leave through the greater sciatic foramen
Anterior rami of S1-S4 + lumbosacral trunk

24
Q

What areas do nerves from the sacral plexus supply?

A

Gluteal, pelvic and perineal regions, and the lower limb (via sciatic nerve)

25
Q

Give the major branches of the sacral plexus, their roots and a mneumonic

A 
Superior gluteal (L4,5,S1)
Inferior gluteal (L5,S1,S2)
Sciatic (L4-S3): splits into tibial and common peroneal
Posterior femoral cutaneous (S1-S3)
Pudendal (S2-S4)

Some Irish Sailor Pesters Polly

26
Q

What does the iliohypogastric nerve innervate (L1 some T12)?

A

Motor-internal oblique and transversus abdominis

Sensory-posterolateral gluteal skin in the pubic region

27
Q

What does the ilioinguinal nerve innervate? (L1)

A

Motor: internal oblique and transversus abdominis
Sensory: skin on upper medial thigh, penis/labium majus

28
Q

Where does the lateral femoral cutaneous nerve innervate?(L2,L3)

A

Enters thigh at lateral surface of inguinal ligament so supplies there and anterior and lateral thigh to the knee

29
Q

What does the obturator nerve innervate? (L2-4)

A

Motor: obturator externus, pectineus, adductor longus, adductor brevis, adductor magnus, gracilis
Sensory: skin over medial thigh

30
Q

What does the femoral nerve innervate? (L2-4)

A

Motor: Illiacus, pectineus, sartorius, all the muscles of quadriceps femoris
Sensory: skin on anterior thigh and medial leg

31
Q

What does the superior gluteal nerve innervate? (L4-S1)

A

Motor: gluteus medius, gluteus maximus, tensor fasciae latae

No sensory

32
Q

What does the inferior gluteal nerve innervate? (L5-S2)

A

Motor: gluteus maximus

No sensory

33
Q

What does the sciatic nerve innervate? (L4-S3)

A

Motor:
Tibial Portion – all of the muscles in the posterior compartment of the thigh, including the hamstring portion of adductor magnus, apart from the short head of the biceps femoris. All muscles in the posterior compartment of the leg. All muscles in the sole of the foot.

Common Fibular Portion – Short head of biceps femoris, all muscles in the anterior and lateral compartments of the leg and extensor digitorum brevis.

Sensory Functions:
Tibial Portion: skin on the posterolateral and medial surfaces of the foot as well as the sole of the foot.

Common Fibular Portion: skin on the anterolateral surface of the leg and the dorsal aspect of the foot.

34
Q

What does the posterior femoral cutaneous nerve innervate? (S1-3)

A

No motor

Sensory: skin on posterior thigh + leg and skin of perinium

35
Q

What does the pudendal nerve innervate? (S2-S4)

A

Motor:skeletal muscles in the perineum, the external urethral sphincter, the external anal sphincter, levator ani
Sensory: penis and the clitoris and most of the skin of the perineum.

36
Q

What is a myotome?

A

A unilateral group of muscles supplied by a single spinal nerve or spinal nerve root
NOT a motor unit (motor unit=motor neurone + the skeletal muscle fibres it innervations). 1 spinal nerve has the neurones of many motor units but 1 spinal nerve supplies a single myotome

37
Q

How is the level of neural injury decided?

A

By the lowest level that is functioning (nerve roots; don’t directly correlate with vertebral levels esp in thoracic and lumbar)

38
Q

What are the movements for each myotome?

A

Learn list/watch video :)

39
Q

What is a dermatome?

A

An area of skin supplied by a single spinal nerve/spinal nerve root. There is functional overlap between adjacent dermatomes, but not over an axial line
Each is named according to the spinal nerve providing most of its innervation

40
Q

What is an axial line?

A

The junction of 2 dermatomes supplied from discontinuous spinal levels:

  • limbs have anterior and posterior axial lines
  • mark the centre of either the pre-axial or post-axial compartments of the limb (e.g. anterior and posterior compartment forearm)
  • boundaries of the pre and post axial compartments marked by veins: cephalic and basilic, great and small saphenous
41
Q

What are peripheral nerve distributions?

A

NOT DERMATOMES
Areas of skin supplied by the peripheral nerves, e.g. the femoral nerve innervates skin on the anterior thigh
Within each peripheral nerve there may be fibres from more than one spinal nerve (e.g. femoral nerve is from L2, L3 and L4

42
Q

What happens in Herpes Zoster virus (shingles)?

A

Almost always affects the skin of a single dermatome
Reactivation of the varicella zoster virus (chickenpox)
Virus travels through a cutaneous nerve and remains dormant in a dorsal root ganglion after chickenpox. When host is immunosuppressed VZV reactivates and travels through peripheral nerve to the skin of a single dermatome

43
Q

Why do lesions of a single posterior root/spinal nerve rarely cause numbness?

A

Because the fibres conveyed by the adjacent spinal nerves almost completely overlap
therefore, dermatome map lines would be better represented as smudges… generally at least 2 adjacent spinal nerves must be interrupted to cause numbness

44
Q

What is Wallerian degeneration?

A

Anterograde degeneration of axons detached from cell bodies: axon and myelin sheath

ESA 2 (58 decks)

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